The present invention relates to a binderless storage phosphor screen with a vapor deposited phosphor layer.
A well known use of storage phosphors is in the production of X-ray images. In U.S. Pat. No. 3,859,527 a method for producing X-ray images with a photostimulable phosphor, which are incorporated in a panel is disclosed. The panel is exposed to incident pattern-wise modulated X-ray beam and as a result thereof the phosphor temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which (are) be processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called xe2x80x9cdigital radiographyxe2x80x9d or xe2x80x9ccomputed radiographyxe2x80x9d.
The image quality that is produced by any radiographic system using a phosphor screen, thus also by a digital radiographic system, largely depends on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be.
This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimum sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by physical vapor deposition, which may be thermal vapor deposition, sputtering, electron beam deposition or other of phosphor material on a substrate. However, this production method can not be used to produce high quality screens with every arbitrary phosphor available. The mentioned production method leads to the best results when phosphor crystals with high crystal symmetry and simple chemical composition are used.
The use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and the high crystal symmetry of these phosphors makes it possible to provide structured screens and binderless screens.
It has been disclosed that when binderless screens with an alkali halide phosphors are produced it is beneficial to have the phosphor crystal deposited as some kind of piles, needles, tiles, etc. So in U.S. Pat. No. 4,769,549 it is disclosed that the image quality of a binderless phosphor screen can be improved when the phosphor layer has a block structure, shaped in fine pillars.
In U.S. Pat. No. 5,055,681 a storage phosphor screen comprising an alkali halide phosphor in a pile-like structure is disclosed. The image quality of such screens still needs to be increased and in JP-A-06/230 198 it is disclosed that the surface of the screen with pillar like phosphors is rough and that a levelling of that surface can increase the sharpness. In U.S. Pat. No. 5,874,744 the attention is drawn to the index of refractivity of the phosphor made use from in order to produce the storage phosphor screen with a needle-like or pillar-like phosphor.
In EP-A-1 113 458 a binderless storage phosphor screen is disclosed that comprises an alkali metal storage phosphor characterized in that said screen shows an XRD-spectrum with a (100) diffraction line having an intensity I100 and a (110) diffraction line having an intensity I110, so that I100/I110xe2x89xa71. Such a phosphor screen shows a better compromise between speed and sharpness.
It is known from U.S. Pat. No. 4,769,549 to use anodized aluminium as a substrate for vapor deposited phosphor layers. The use of anodized aluminium is said to have the advantage that the Al2O3 on the anodized aluminium is present as a kind of tiles. It is said that, when vapor depositing a storage phosphor layer on such a substrate, the phosphor forms in fine pillar-shaped blocks, separated from each other by voids, and that, by having such a storage phosphor layer, a very good speed/sharpness relation is realized.
The preparation of anodized aluminium however presents the drawback that a large amount of electric energy is typically required for roughening and oxidising the substrate surface. In addition, the roughening achieved by etching can generally only be carried out relatively slowly. A further disadvantage is that reprocessing of the waste products formed during anodising and during roughening of the substrate is expensive. All elements taken together make anodized aluminium a quite expensive substrate for a phosphor screen and thus the demand for a less expensive substrate that preserves the advantages of anodized aluminium is desired.
It is an object of the present invention to provide a stimulable phosphor screen useful in an X-ray recording system with a very good compromise between speed of the recording system (i.e. as low as possible patient dose) and with an image having high sharpness and low noise.
It is a further object of the invention to provide a stimulable phosphor screen useful in an X-ray recording system with a very good compromise between speed of the recording system (i.e. as low as possible patient dose) with an image with high sharpness and low noise on a quite inexpensive substrate.
The above mentioned object is realized by providing a stimulable phosphor screen having the specific features defined in claim 1. Specific features for preferred embodiments of the invention are disclosed in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.